Gegevenstypen converteren

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Tyler Pernes

Learning & Development Consultant

Hoofddoel

‘Rommelig’ gegevenstypen

Probleem 1: Type-specifieke functies

Probleem 2: Tabellen combineren (JOIN)

Probleem 2: Tabellen combineren (UNION)

Fouten interpreteren

Fout: type-specifieke functie

SELECT AVG(first_name)
FROM athletes;

ERROR: Function avg(character varying) does not exist

JOIN-fout:

SELECT country, continent
FROM countries AS c1
JOIN continents AS c2
ON c1.continent_id = c2.id;

ERROR: Operator does not exist: integer = character varying

Oplossing: wikkel in CAST()

Syntaxis:

CAST(field AS type)

Voorbeelden:

CAST(birthday AS date)
CAST(country_id AS int)

CAST voor functies

SELECT DATE_PART('month',birthdate)
FROM birthdates;

ERROR: Can't run DATE_PART on string.

SELECT DATE_PART('month', 
       CAST(birthdate AS date)) 
FROM birthdates;

+-----+
| 04  |
| 05  |
+-----+

CAST voor JOINs

SELECT a.id, b.id
FROM table_a AS a
JOIN table_b AS b
ON a.id = b.id;

ERROR: Cannot join ON varchar = int.

SELECT a.id, b.id
FROM table_a AS a
JOIN table_b AS b
ON a.id = CAST(b.id AS varchar);

Query Ran Successfully!

Plannen voor typeproblemen

Los ze op zodra ze verschijnen
Lees foutmeldingen!

Plannen voor typeproblemen

SELECT column_name, data_type
FROM information_schema.columns
WHERE table_name = 'countries';

+--------------+--------------------+
| column_name  | data_type          |
|--------------|--------------------|
| id           | integer            |
| country      | character varying  |
| region       | character varying  |
+--------------+--------------------+

Documentatie gegevenstypen

https://www.postgresql.org/docs/9.5/datatype.html

Laten we oefenen!

Rapporteren in SQL